Existing thermo-compression bonding systems are quickly becoming inadequate as higher demands are placed on such systems by new and rapidly improving technology. Improvements to current thermo-compression bonding systems are required in order to facilitate thermo-compression bonding of the latest and largest dies and substrates. Some key components of such systems, including a description of their capabilities and limitations, are outlined below.
Co-Planarity Adjustment System
In high accuracy thermo-compression bonding applications it is critical to ensure co-planarity of the surfaces of the die and substrate to be bonded. Co-planarity, as used in this disclosure should be read as two opposing surfaces which are parallel to one another, i.e. they occupy the same plane. Attaining co-planarity of the surfaces upon which these components rest is the first step in achieving a uniform bonding of die to substrate; when the die and substrate are perfectly parallel, all solder bumps on the die will make contact with their corresponding substrate bond pads simultaneously, assuming all solder bumps were perfectly formed. Lack of co-planarity results in poor bonding with an uneven gap between die and substrate around the die perimeter once the bonding is completed. This can lead to the bonding process being compromised in the extreme case. In other cases the downstream process step of under-filling the gap between die and substrate is compromised, resulting in yield losses.
Prior art co-planarity adjustment mechanisms tend to be unduly complicated and do not typically ensure that the corresponding surfaces are parallel across their entire surfaces. One example of such a prior art system is an auto-collimator. An auto-collimator is expensive and complex, requiring a high quality objective lens, capable of producing a collimated beam (parallel light rays) and motors with fine degrees of control, while only providing a point measurement, or the measurement of multiple points. Auto-collimators work by collimating light, using a high quality objective lens and directing the collimated light onto a plane mirror. The separation between the original (i.e. incident) light and the reflected light is then compared. By comparing the separation of the reflected light and adjusting the angle of the working surfaces, the working surfaces can be brought into a parallel relationship. In the foregoing description, a plane mirror should be understood as a mirror having a planar reflective surface. For light rays striking a plane mirror, the angle of reflection equals the angle of incidence, enabling the foregoing measurements. Importantly, a collimated beam of light does not spread out after reflection from a plane mirror, except for diffraction effects. Measurements of co-planarity made using this method are only based on a single point or a small number of single point measurements taken at various locations, however, making this method not particularly well suited to ensuring parallelism of opposing planes across their entire surfaces requiring extreme accuracy.
Therefore, it is desirable to have an automatic method of ensuring the surfaces that hold the die and substrate are parallel to one another, using the average across the entire surface as the reference.
Gantry Without Moment Loading
Gantries are commonly used in many automated processes to move parts from one place to another with precision. In most gantry designs used for thermo-compression bonding, the placement head is offset in an X or Y axis from the gantry rail/bearing supports, assuming bonding force is applied along a Z-axis and all planes are orthogonal to one another. Using such a gantry to apply force to a part however requires that the force applied not cause an excessive amount of deflection in the gantry itself, which is typically limited by the load carrying ability of the bearings upon which it rides.
For lower force applications, the moment loads exerted (by the placement head during die placement) on the gantry bearings are negligible. At higher forces however, the moment loads can be significant. For applications requiring both high forces and high accuracy, these moment loads can result in small excursions at the bearing support, which are then amplified at the placement head, since they are offset in X or Y axis from the bearing. This results in placement inaccuracies. High force bonding, in this context, typically involves applying a bonding force of approximately 30-50 Kg.
Prior art devices would typically use a conveyor belt, moving table, or other system to bring parts to a high force bonding system, which was stationary and which would typically make use of a large welded structure as a support. This structure added complexity and cost to the system and reduced the room available for other subsystems within the thermo-compression bonding system as well as the flexibility of the gantry itself, making a gantry capable of high force application without moment loading commercially desirable. Such a system would ideally allow for 6 degrees of freedom in the thermo-compression bonding system, but would enable the locking out of five of those degrees of freedom during high force bonding.
Heating & Cooling System
Thermo-compression bonding requires heating of a die and substrate in order to initiate bonding. Typically, in flip-chip thermo-compression bonding, the die will have already had small solder bumps applied to the surface to be bonded to the substrate, which has typically has small metal pads applied to the side to be bonded to the die that correspond to the solder bumps on the die. The die and substrate must be heated while pressure is applied in order to bond the die to the substrate. Even a single failed bond can cause a malfunction of the product. Die head heaters are used to heat the die, while substrate heaters are used to heat the substrate.
Die head heaters used in thermo-compression bonding require high temperature ramp and cooling capability as well as temperature uniformity to ensure reliable thermo-compression bonding. These heaters are typically run at 400-420° C., but may be run to 500° C. in some situations. Substrate heaters are typically kept at lower temperature (˜150° C.) and may not require the high temperature ramp/cooling capability of the bond head heater.
During thermo-compression bonding, heat is typically applied primarily to the die (typically silicon) since it can handle higher temperatures than the substrate, which is typically made out of organic material. This heat is transferred from the die to the substrate via the aforementioned solder bumps. The substrate acts as a heat sink, pulling heat away from the die. The attach process naturally creates a hot spot at the center of the die whereby the center solder bumps fuse to the substrate pads below, but the bumps at the outer periphery of the die may not. Typically, the larger the die, the larger the temperature gradient between the center and the edge of the die will be. This is true even with a heater which is uniformly heated across the die surface. This may lead to poor attach at the die edges, which is especially problematic in the case of relatively large dies.
To achieve adequate bonding at the edges of the die using such a system, the die head temperatures may be set higher than would otherwise be required. This exposes portions of the die to higher temperatures than required, which could lead to failure of the die or reliability issues later. The extra heat required by this uneven bonding must also be generated and dissipated during production of each die/substrate combination, resulting in longer cycle times than might otherwise be achievable.
Another concern regarding thermo-compression bonding heaters is their ability to heat and to dissipate heat quickly. Lowering the ramp-up and ramp-down times are crucial for lowering the overall cycle time and enabling higher production rates. Prior art systems use relatively small heaters with forced air cooling. This can work well for smaller dies, but results in unacceptably long cycle times for larger dies.
Current workarounds involve the use of trim heaters, which attempt to balance the extra heat loss from the edges by adding a fixed amount of heat back into the edges of the heater. This can also minimize thermal stresses by minimizing uneven expansion. Even with the current state of the art, the centers of such heaters still tend to be relatively hot as compared to other portions of the heater.
Still another concern regarding heating of a die and substrate in thermo-compression bonding applications is that, during bonding, the exact temperature at the intermetallic interface between the die bumps and the substrate pads is not known and not easy to determine in a dynamic system. In prior art systems, a delay is introduced after the bond head reaches a pre-determined temperature, to ensure the desired die bumps to substrate pad interface temperatures are reached where the bumps melt, alloy and attach to the substrate pad. These delays are empirically determined for a given die/substrate combination, resulting in shorter or longer cycle times and potential under or over heating of the die and substrate. This could be avoided if there was a direct method of determining that bonding had occurred or the pre-determined temperature had been reached.
Furthermore, the substrate heater arrangement is typically larger than that of the die heater, to cater to a wide range of substrate sizes, and has significant thermal mass. When the die and substrate are brought together and the die is subsequently heated, the substrate with the heater arrangement below acts as a heat sink, pulling heat away. Since the mass of the substrate heater is significantly higher, its temperature does not appreciably change during the bond cycle. It also means that the bond head, typically above the substrate head, must have enough heating power to overcome the ‘drag’ of the heat sink effect of the substrate heater.
Thus for higher performance, throughput and bonding of larger dies a solution is needed to minimize temperature gradients during the attachment phase of thermo-compression bonding and to allow for more accurate judgment of temperature at the interface between die bumps and substrate pads.
Bond Chamber Sealing:
During the thermo-compression bonding process, it is often necessary to evacuate the bonding chamber of oxygen and other oxidizing agents because of the elevated temperatures used, which may result in unacceptable degradation of the product if not addressed. Typically, nitrogen is used as the inert gas, but other fluids, gases or combinations of either or both are also known. Occasionally, fluids, gases or combinations of either or both which are capable of removing oxidation, especially at lower temperatures, are used as well.
In the first case, it is desirable to form a sealed bonding chamber to make efficient use of the inert gas. In the second case, it is typically imperative to seal the chamber to prevent the fluid or gas from reacting with other components of the machine, possibly causing degradation, as well as to protect the health and safety of the operators of such machines. Typically, an oxygen sensor would be used to ensure that the chamber was sufficiently evacuated for the intended operation to proceed.
Current machines have not yet devised a solution which provides efficient sealing while retaining some flexibility to conform to a surface that is not precisely co-planar. Current solutions also fail to offer a seal which may retain its sealing ability while engaging in limited sliding motion in the plane defined by a sealing surface on the substrate head.
What is needed, therefore, are techniques for sealing a bonding chamber which allow for the efficient usage of inert and oxide removal fluids, gases or combinations of either or both, which can provide effective sealing between surfaces which may not be co-planar while allowing sliding motion in the plane defined by a sealing surface of the substrate head.